1. Field of the Invention
The present invention relates to a recording medium and an image forming process using this recording medium, and particularly to a recording medium comprising a porous cellulose layer of a low density, or a recording medium, in which an ink receptive layer is provided on a substrate composed of a porous cellulose layer of a low density. More particularly, the present invention relates to a recording medium, which can provide a clear and high-quality recorded image and can relieve a phenomenon called cockling wherein a printed surface is waved by an aqueous ink.
2. Related Background Art
In recent years, an ink-jet recording system, in which minute droplets of ink are caused to flow by any one of various working principles to apply them to a recording medium such as paper, thereby making a record of images, characters and/or the like, has frequently been used. A recording apparatus of this system has such a feature that printing can be conducted at high speed and with a low noise, color images can be formed with ease, printing patterns are very flexible, and development and fixing process are unnecessary. Therefore, it has been quickly spread as a recording apparatus for various images in various applications including information instruments. Further, images formed by a multi-color ink-jet system are comparable with those of multi-color prints with a plate making system and photoprints with a color photographic system. Therefore, such images can be obtained at lower cost than the usual multi-color prints and photoprints when the number of copies is small. It thus begins to be widely applied to a field of recording of full-color images.
With the enlarged utilization of the ink-jet recording system, further improvements in recording properties such as speeding up and high definition of recording, and full-coloring of images are required, so that recording apparatus and recording methods have been improved up to date. On the other hand, recording media have also been required to have higher properties. More specifically, the recording media are required to have the following properties:
a) providing printed dots high in density and bright and vivid in color tone upon forming images;
b) having high ink absorption rate and absorption capacity so as for an ink not to run out or bleed in case printed dots overlap each other;
c) preventing printed dots from diffusing in a lateral direction beyond need;
d) providing printed dots having a substantially round shape, and smooth and clear in periphery, and
e) having high whiteness degree and glossiness.
In order to meet such requirements, a wide variety of recording media have heretofore been proposed. For example, there has been proposed ink-jet recording paper, in which a coating layer having good ink absorbency is provided on a surface of a substrate (see Japanese Patent Application Laid-Open No. S55-005830). There has been also proposed the use of amorphous silica as a pigment in an ink-receiving layer laminated on a substrate for recording medium (see Japanese Patent Application Laid-Open No. S55-005158).
With the diversification of uses of recording media, it has also been required to reduce the occurrence of curling or cockling of printed articles for the purpose of improving the quality of recorded images. In the present invention, cockling means a phenomenon that a printed surface of a recording medium is made irregular or waved.
As means for avoiding this cockling phenomenon, there have heretofore been proposed the following methods.
(1) Japanese Patent Application Laid-Open Nos. H03-038376, H03-199081, H07-276786 and H08-300809 describe recording media using paper having an underwater elongation and a wetted elongation within respective specified ranges.
(2) The constitutions in which an ink-receptive layer containing a water-repellent component (Japanese Patent Application Laid-Open No. 2000-158805) or a void layer formed of a thermoplastic resin such as polyurethane (Japanese Patent Application Laid-Open No. 2002-154268) are respectively provided as intermediate layers for barrier preventing penetration of ink between an ink-receiving layer and a substrate is described.
(3) Proposals for the solution, which are different from the methods in the above-described publicly known documents, include the following proposals. Namely, the proposals comprise providing an additional structure on a recording medium. A recording medium, in which ink-receptive layers are provided on both surfaces of a substrate, a recording medium, in which a back coat layer is provided on a surface opposite to an ink-receiving layer, and a recording medium, in which substrates are laminated on each other into a two-layer structure, are described in Japanese Patent Application Laid-Open Nos. H02-270588, 2001-253160 and 2002-002092, respectively.
Since the technical ideas described in Japanese Patent Application Laid-Open Nos. H03-038376, H03-199081, H07-276786 and H08-300809 are based on the premise that water is evenly given to the whole part of a recording medium, however, they cannot cope with a case where liquids different in properties are applied to every part like ink-jet recording. In addition, since the intermediate layers described in Japanese Patent Application Laid-Open Nos. 2000-158805 and 2002-154268 both act as a barrier which prevents penetration of ink, the ink printed do not penetrate into the substrate when the quantity of ink printed is great. As a result, the quantity of ink absorbed is reduced, and an ink-absorbing rate is lowered, so that ink overflowing and/or bleeding may be caused in some cases.
The present inventor has carried out an investigation on various kinds of the recording media proposed in the prior art documents mentioned above and found that on all the recording media, the following problems are involved.
(1) The effect to prevent cockling may have not been obtained in some cases according to the basis weight and thickness of the recording medium. In particular, cockling has markedly occurred when the thickness of the recording medium is as thin as 150 μm or smaller. More specifically, this is attributable to the circumstance that since the recording medium is swollen by ink absorption, and causes shrinkage in a drying step, the stiffness of the recording medium is lowered when the thickness of the recording medium is thin, so that the degree of deformation by the swelling and shrinkage of the recording medium becomes great. As described above, it has been found that cockling cannot be effectively inhibited in recording media low in basis weight and recording media thin in thickness according to the conventional methods.
(2) When the surface of a recording medium has been smoothed by a calendering treatment, the occurrence of cockling has markedly increased. This is attributable to the circumstance that the properties (three-dimensional configuration of cellulose fiber, pore structure between cellulose fibers, etc.), and the like of cellulose making up the recording medium are changed by the smoothing treatment. As described above, it has been found that the occurrence of cockling increases when the surface of the recording medium is smoothed for improving image quality, and any recording medium capable of attaining both improvement of image quality and inhibition of occurrence of cockling at the same time cannot be provided.
(3) When an ink-receiving layer has been formed on a recording medium, in which the underwater elongation of base paper has been controlled like Japanese Patent Application Laid-Open Nos. H03-199081 and H08-300809, the cockling phenomenon caused by ink has become marked compared with a recording medium on which no ink-receiving layer has been formed. This is attributable to the circumstance that the underwater elongation cannot be controlled due to absorption and drying of water in the recording medium in a step of forming the ink-receiving layer. As a result, it has been found that when an ink-receiving layer is formed, cockling may not be inhibited in some cases even when the underwater elongation has been controlled in a papermaking step.
Further, the present inventor has found that when an image is formed while increasing the amount of an ink applied to a recording medium to 2 times or 3 times, the ink-absorbing capacity of the recording medium itself is lowered, ink overflowing and/or bleeding may be caused in some cases to fail to achieve good image quality.
It has also been confirmed that when an image is formed on various kinds of the recording media proposed in the prior art documents by a printer for conducting high-speed printing in recent years, it is not always satisfactory from the viewpoints of image quality, surface gross, curling, cockling, paper conveyability and the like.
The phenomena of curling and cockling are both considered to be caused by occurrence of expansion and contraction and/or distortion in a recording medium by ink absorption. The cause of these phenomena will hereinafter be described in detail. A condition where cellulose has been dispersed in a beating liquid after pulp used in a conventional recording medium has been beaten is illustrated in FIG. 3A. When the pulp is beaten, the fiber length of cellulose 22 making up the pulp is shortened as shown in FIG. 3A, and at the same time fibrillation (to cause branching of fiber) progresses, so that a great number of branches 21 are produced to increase the surface area of the cellulose.
Then, a condition where paper has been made with the cellulose after the beating to produce a recording medium is illustrated in FIG. 3B. Incidentally, FIG. 3B partially shows a microstructure of the recording medium. Since the cellulose 22 has a large surface area, hydrogen bonds (indicated by a dotted line) are formed at many positions. As a result, the volume of pores in the interior of the recording medium decreases to provide a recording medium high in density.
When printing is then conducted on this recording medium, an ink component is absorbed in the interior of cellulose and between cellulose fibers. As a result, the hydrogen bonds formed at many positions between cellulose fibers are cleaved by water and a hydrophilic component 23 contained in the ink as illustrated in FIG. 4A. The cellulose itself is also deformed by absorption of water and the like.
When the recording medium, in which the ink has been absorbed, is then dried, water and the hydrophilic component bonded to the cellulose are removed, and hydrogen bonds are formed again between the cellulose fibers. At this time, the cleavage (FIG. 4A) of the hydrogen bonds formed between the cellulose fibers by the ink absorption and the formation (FIG. 4B) of the hydrogen bonds between the cellulose fibers by the drying are not conducted at exactly the same positions (hydrogen bonds are formed at positions different from the positions where the original hydrogen bonds have been formed). Therefore, the recording media shown in FIGS. 4B and 3B are different in positions of the hydrogen bonds have been formed, and thus both recording media have different spatial configurations from each other. The cause of this is considered to be attributable to the circumstance that evaporation of ink components upon the drying is not conducted completely evenly at all portions within the recording medium, and that the cellulose itself is deformed by the ink absorption. Such a phenomenon is considered to appear as a cockling phenomenon as the whole of the recording medium.
The present inventor has thus carried out an extensive investigation. As a result, it has been discovered that there is a need to produce a recording medium having the following properties for solving such problems as described above:
(a) being high in the stiffness of fibers making up the recording medium, and little in deformation attendant on the absorption and drying of ink; and
(b) being not changed in the relative spatial configuration of the fibers making up the recording medium upon the absorption and drying of ink, calendering or the like (having no change in the pore structure between fibers).
The present inventor has found that in order for the resulting recording medium to have the above-described properties (a) and (b), it is only necessary to use cellulose subjected to a particular treatment and to fill a porous filler into pores formed by making the density of a porous cellulose layer formed of this cellulose low.
The present inventor has further found that the porous filler is filled into the pores in a particular filled state, whereby deformation of the cellulose attending on the absorption and drying of the ink or calendering is effectively assimilated within voids formed between particles of the porous filler to more hardly cause curling, cockling and the like.
In other words, the present invention has the following objects.
It is a first object of the present invention to provide a recording medium, which has the above-described constitution, and is good in ink absorbency and little in the frequency of occurrence of cockling even when the recording medium is composed of a thin paper having a thickness of 150 μm or smaller.
A second object of the present invention is to provide a recording medium, which is free from ink overflowing, provides images high in density and bright or vivid in color tone and inhibits cockling even when the surface of the recording medium is subjected to a smoothing treatment.
A third object of the present invention is to provide a recording medium, which does not increase the frequency of occurrence of cockling even when an ink-receiving layer is provided on a substrate, compared with a recording medium comprising no ink-receiving layer provided on the substrate.